For example, in a conventional image encoding device described in the following nonpatent reference 1, an inputted color image is partitioned into largest coding blocks each having a predetermined size, and each largest coding block is further partitioned hierarchically into smaller coding blocks.
Each coding block is further partitioned into smaller prediction blocks, and an intra-screen prediction and a motion-compensated prediction are performed on each of the prediction blocks to generate a prediction error.
Further, the prediction error is divided hierarchically into transformation blocks within each coding block, and each of the transform coefficients is entropy-encoded, thereby achieving a high compression ratio.
In order to implement efficient encoding using a temporal correlation in a conventional image encoding device, a block having a high correlation with a coding target block is searched for from an already-encoded picture on the basis of, for example, an encoding configuration as shown in FIG. 37, and encoding using a motion-compensated prediction which provides the block which is searched for as a predicted value is implemented. Generally, the picture through which the search (reference) is performed at that time is referred to as a reference picture. Particularly, it is known that a bidirectional prediction which is motion compensation which refers to both a past picture and a future picture in display order as shown in FIG. 37 can implement a high-accuracy prediction. However, providing a reference relation between pictures in this way results in dependence occurring in the decoding of each picture, and, as a result, a halfway playback of a sequence, this playback decoding an encoded bitstream from some point of this bitstream, cannot be implemented.
Therefore, when using the encoding configuration using a bidirectional motion-compensated prediction as shown in FIG. 37, there is a case of preparing a random access point at which to make it possible to correctly perform a playback even if the encoded bitstream is decoded from some point of the bitstream. For example, a case is examined in which a gray (shaded) picture having a number of 8 in the display order of FIG. 37 (having a number of 1 in the decoding (encoding)) order is set as a picture which is described in nonpatent reference 1 and can be accessed randomly (Intra Random Access Point (IRAP) picture described in nonpatent reference 1). In nonpatent reference 1, a picture whose decoding order (decoding order in a decoding device, and this decoding order has the same meaning as the encoding order in an encoding device) is later than that of an IRAP picture and whose display order is earlier than that of the IRAP picture (each of pictures having numbers 1 to 7 in the display order of FIG. 37) is defined as a “leading picture”, and a picture whose decoding order and display order are later than those of the IRAP picture (each of pictures having numbers 9 to 16 in the display order of FIG. 37) is defined as a “trailing picture.” When the gray (shaded) picture having a number of 8 in the display order of FIG. 37 (having a number of 1 in the decoding (encoding) order) is a CRA (Clean Random Access) picture which is a kind of an IRAP picture, there is no guarantee that the leading pictures can be decoded correctly when the decoding is started from the CRA picture, but the trailing pictures can be decoded correctly at all times. Limitations imposed on each associated picture are defined so that this operation is guaranteed. Concretely, in the image encoding device described in nonpatent reference 1, the use of each leading picture as a reference picture of a trailing picture is prohibited, while the existence of a picture whose display order is later than that of an IRAP picture and whose decoding order is earlier than that of the IRAP picture is also prohibited. In addition, in nonpatent reference 1, it is defined that each leading picture must be decoded (encoded) before the trailing pictures. Under such a definition, also when starting the decoding at some point of the encoded bitstream, by using a CRA picture, a picture whose display order is later than that of the CRA picture can be always decoded correctly by starting the decoding from the CRA picture, and a halfway playback of the encoded sequence can be implemented. Further, because it is only guaranteed that the trailing pictures can be decoded correctly in the case in which the picture is a CRA picture, the leading pictures make it possible to perform a bidirectional prediction also including the CRA picture, and a reduction of the coding efficiency which is caused by the insertion of the picture which can be accessed randomly can be reduced.